1. Field of the Invention
The present invention relates to a protective circuit for limiting the voltage for a device to be protected, particularly a load with an upstream-connected voltage and current limiting device. Such a protective circuit has in each case one input terminal and one output terminal, a common line and a voltage detector connected between the input terminal and the common line. The device to be protected is located in an output circuit of the protective circuit and is separable by means of a controllable switching device provided in a serial arrangement between the input terminal and output terminal and in which the voltage detector is in operative connection with the controllable switching device.
2. Description of the Related Art
Protective circuits of this type are known from EP 359 912 A1, EP 310 280 B1 or German patent 36 22 268. These known protective circuits suffer from various disadvantages. Thus, e.g. following the response of the corresponding fuse mechanism in EP 359 912 A1 it is necessary to manually replace a fuse. The circuits of German patent 36 22 268 and EP 310 280 B1 suffer from the disadvantage of high shunt currents and in addition an undesired voltage drop occurs in the operating area. Another disadvantage is that in the case of a fault a high power loss can arise.
Although in the not previously published PCT application WO 00/62394 A1 a solution is obtained to the aforementioned problems in that a protective circuit is placed upstream of the fuse mechanism for the load and has a rapid response behaviour, so that in the case of an overvoltage or an excessive current a rapid switching off of the downstream circuits is possible. However, the switch in this case is constituted by a FET located in the longitudinal branch between an input terminal of the protective circuit and the downstream fuse mechanism.
When using an FET the wiring possibilities and the arrangement in the protective circuit are limited and relatively inflexible, so that improvements are needed.
DE 38 04 250 C1 describes a circuit arrangement for current limiting purposes, particularly for use in the case of digital terminal devices. For switching off a voltage in a longitudinal branch of the circuit arrangement is provided the source-drain path of a field effect transistor, whose passage is controlled by its gate as a function of both the turning on of a transistor and the charging of a capacitor.
DE 296 13 790 U1 relates to a micromechanically manufactured microswitch in which, as comparable with a reed contact, a switching movement can be brought about by a magnetic field acting on an elastic, bendable support.
DE 41 00 634 A1 discloses a test device for IC's, particularly in mounted printed circuit boards, in which for the connection of individual test points of a testpiece with different test channels a plurality of microrelays is provided.